To impart various properties, such as water resistance and heat sealability, to a container made of paper (paper container), it has been common practice to apply a resin coating to a paper base material.
In such a laminate with a resin coating applied to a paper base material, a polyolefin such as polyethylene or polypropylene is generally used as a coating resin, because it is excellent in workability, and also excellent in adhesion to paper and melt extrudability. However, polyolefin is not fully satisfactory in terms of heat resistance, properties as a barrier against an offensive odor, aroma barrier properties, etc. which are necessary functions for food containers, so that its uses are limited.
To solve such problems, a proposal has been made to provide a paper base material with a coating comprising a polyester resin. However, coating a homopolyethylene terephthalate resin (may hereinafter be referred to as “homo-PET”), which is a general-purpose polyester, on a paper base material directly, not via an adhesive layer, poses difficulty in obtaining stable film-forming properties. A laminate having homopolyethylene terephthalate coated directly on a paper base material has not been commercialized.
Thus, Patent Document 1 to be described later, for example, proposes a method for producing a laminate, which comprises extrusion-coating a polyester on a fibrous base material, wherein a polyester consisting essentially of ethylene terephthalate units and having a melt viscosity ratio η50/η950 (η50 is a melt viscosity at 280° C. and a shear rate of 50 s−1, and η950 is a melt viscosity at 280° C. and a shear rate of 950 s−1) in the range of 2.0<η50/η950<3.0 and an inherent viscosity in the range of 0.5 to 1.4 dl/g is used as the polyester, and the extrusion coating is performed under the following conditions: a die downward extrusion temperature of 300 to 340° C., a melt extension ratio of 25 or less, a coating thickness of 20 μm or more, and a roll application line pressure of 5 to 50 kg/cm. This method is described as being capable of producing, with high stability, a polyester coated paper having a uniform film thickness and excellent interlaminar bonding strength.
Another proposal has been made for a multilayer polyester-based laminated paper having polyethylene terephthalate laminated to a paper base material via a polyethylene terephthalate copolymer resin having a density of less than 1.35 g/cm3 (Patent Document 2).